CuA, an electron transfer center present in cytochrome c oxidase, COX, and nitrous oxide reductase, N2OR, is a dimeric copper complex with four ligands, two cysteine thiols bridging the metal ions and two terminal histidine residues. The center cycles between the mixed-valence state [Cu(I),Cu(II)] and the reduced state [Cu(I),Cu(I)]. The EPR, optical absorption, low-temperature magnetic circular dichroism, and CD spectra of three proteins containing the mixed-valence state of CuA have been measured between 33 000 and 5000 cm-1. These results point to two forms of the chromophore, one in the enzyme N2OR of Pseudomonas stutzeri, lacking its catalytic center, and also in a water soluble domain of subunit II of Paracoccus denitrificans COX and the other, referred to as CuA*, in a site engineered into a soluble domain of subunit II of the quinol oxidase in Escherichia coli. An assignment of the electronic spectrum has been made in terms of a covalent planar core [Cu2(SR)2]+ with a Cu−S distance of 2.2 A, a Cu−...